1. Field of the Invention
The present invention relates to a radiation image detection method and apparatus for detecting a radiation image from a radiation image detector which includes multitudes of pixels, each having a TFT switch, disposed two-dimensionally.
2. Description of the Related Art
Radiation image detection systems having a TFT (Thin Film Transistor) active matrix array on which a radiation sensitive layer is disposed are known. Such systems are capable of tentatively storing latent image charges generated according to radiation transmitted through a subject and reading out the stored latent image charges as electrical signals, and have an advantage over conventional imaging plates using a storage phosphor in that they allow instantaneous image verification and motion image monitoring, so that they are spreading rapidly.
First, the structure of a conventional radiation image detection system will be described with reference to FIG. 8. FIG. 8 is a schematic equivalent circuit of 3×3 pixels. In FIG. 8, the reference numerals 111, 112, 113, and 114 respectively denote a capacitor, a TFT switch, a scanning line, and a data line.
Radiation, such as X-rays or the like, enters from a direction normal to the surface of FIG. 8, which is converted to electrical signals by the photoelectric conversion elements and the charges thereof are stored in the capacitor 111 of each pixel. Thereafter, the TFT switches 112 are sequentially activated by the scanning lines 113 to transfer the stored charges to the data lines 114 connected to either one of the source/drain electrodes of the TFT switches 112, and the signals flowing out through the data lines are detected by the signal detectors 115, which are then converted to digital signals by the A/D converter 116 and outputted.
In such type of radiation image detection system, there may be cases in which various types of noise are added to the essential image signals to be detected due to various reasons. For example, one type of noise is caused by leak current of the TFT switch. It is preferable that no leak current flows through the TFT switch that selects a pixel to be detected while it is in OFF state. But, irradiation of a larger amount of X-rays causes a larger amount of charges to be generated in the charge generation layer, which causes the drain voltage to become high, resulting in a large amount of leak current to flow and added to the image signals. In order to solve this problem, for example, Japanese Unexamined Patent Publication No. 2003-319264 proposes a method in which the leak current is read out while the TFT switches are in OFF state, and digital image signals are corrected using the leak current value.
The method described in the aforementioned patent publication, however, reduces, in effect, a usable dynamic range. More specifically, the image signal correction is performed using a process for subtracting the leak current component from the obtained digital image signal, so that the possible value range of the corrected image signal is reduced by the amount corresponding to the amount of the leak current from the dynamic range at the output of the A/D converter. This causes a problem, in particular, where a large amount of X-rays is irradiated, since the leak current becomes great in such a case.
In view of the circumstances described above, it is an object of the present invention to provide a radiation image detection method and apparatus capable of appropriately correcting the image signal by the amount corresponding to the amount of the error caused by leak currents, and preventing the reduction in the dynamic range of the image signal after correction.